The present invention relates to supported polycrystalline diamond (PCD) compacts made under high temperature/high pressure (HT/HP) processing conditions, and more particularly to supported PCD compacts having improved shear strength and impact resistance properties wherein a polycrystalline cubic boron nitride (PCBN) interlayer is interposed between a PCD compact layer and a cemented metal carbide support layer.
A compact may be characterized generally as an integrally-bonded structure formed of a sintered, polycrystalline mass of abrasive particles, such as diamond or CBN. Although such compacts may be self-bonded without the aid of a bonding matrix or second phase, it generally is preferred, as is discussed in U.S. Pat. Nos. 4,063,909 and 4,601,423, to employ a suitable bonding matrix which usually is a metal such as cobalt, iron, nickel, platinum, titanium, chromium, tantalum, or an alloy or mixture thereof. The bonding matrix, which is provided at from about 10% to 30% by volume, additionally may contain a recrystallization or growth catalyst such as aluminum for CBN or cobalt for diamond.
For many applications, it is preferred that the compact is supported by its bonding to substrate material to form a laminate or supported compact arrangement. Typically, the substrate material is provided as a cemented metal carbide which comprises, for example, tungsten, titanium, or tantalum carbide particles, or a mixture thereof, which are bonded together with a binder of between about 6% to about 25% by weight of a metal such as cobalt, nickel, or iron, or a mixture or alloy thereof. As is shown, for example, in U.S. Pat. Nos. 3,381,428; 3,852,078; and 3,876,751, compacts and supported compacts have found acceptance in a variety of applications as parts or blanks for cutting and dressing tools, as drill bits, and as wear parts or surfaces.
The basic HT/HP method for manufacturing the polycrystalline compacts and supported compacts of the type herein involved entails the placing of an unsintered mass of abrasive, crystalline particles, such as diamond or CBN, or a mixture thereof, within a protectively shielded metal enclosure which is disposed within the reaction cell of a HT/HP apparatus of a type described further in U.S. Pat. Nos. 2,947,611; 2,941,241; 2,941,248; 3,609,818; 3,767,371; 4,289,503; 4,673,414; and 4,954,139. Additionally placed in the enclosure with the abrasive particles may be a metal catalyst if the sintering of diamond particles is contemplated, as well as a pre-formed mass of a cemented metal carbide for supporting the abrasive particles and to thereby form a supported compact therewith. The contents of the cell then are subjected to processing conditions selected as sufficient to effect intercrystalline bonding between adjacent grains of the abrasive particles and, optionally, the joining of the sintered particles to the cemented metal carbide support. Such processing conditions generally involve the imposition for about 3 to 120 minutes of a temperature of at least 1300.degree. C. and a pressure of at least 20 kbar.
As to the sintering of polycrystalline diamond compacts or supported compacts, the catalyst metal may be provided in a pre-consolidated form disposed adjacent the crystal particles. For example, the metal catalyst may be configured as an annulus into which is received a cylinder of abrasive crystal particles, or as a disc which is disposed above or below the crystalline mass. Alternatively, the metal catalyst, or solvent as it is also known, may be provided in a powdered form and intermixed with the abrasive crystalline particles, or as a cemented metal carbide or carbide molding powder which may be cold pressed in to shape and wherein the cementing agent is provided as a catalyst or solvent for diamond recrystallization or growth. Typically, the metal catalyst or solvent is selected from cobalt, iron, or nickel, or an alloy or mixture thereof, but other metals such as ruthenium, rhodium, palladium, chromium, manganese, tantalum, and alloys and mixtures thereof also may be employed.
Under the specified HT/HP conditions, the metal catalyst, in whatever form provided, is caused to penetrate or "sweep" into the abrasive layer by means of either diffusion or capillary action, and is thereby made available as a catalyst or solvent for recrystallization or crystal intergrowth. The HT/HP conditions, which operate in the diamond stable thermodynamic region above the equilibrium between diamond and graphite phases, effect a compaction of the abrasive crystal particles which is characterized by intercrystalline diamond-to-diamond bonding wherein parts of each crystalline lattice are shared between adjacent crystal grains. Preferably, the diamond concentration in the compact or in the abrasive table of the supported compact is at least about 70% by volume. Methods for making diamond compacts and supported compacts are more fully described in U.S. Pat. Nos. 3,141,746; 3,745,623; 3,609,818; 3,850,591; 4,394,170; 4,403,015; 4,797,326; and 4,954,139.
As to polycrystalline CBN compacts and supported compacts, such compacts and supported compacts are manufactured in general accordance with the methods suitable for diamond compacts. However, in the formation of CBN compacts via the previously described "sweep-through" method, the metal which is swept through the crystalline mass need not necessarily be a catalyst or solvent for CBN recrystallization. Accordingly, a polycrystalline mass of CBN may be joined to a cobalt-cemented tungsten carbide substrate by the sweep through of the cobalt from the substrate and into the interstices of the crystalline mass notwithstanding that cobalt is not a catalyst or solvent for the recrystallization of CBN. Rather, the interstitial cobalt functions as a binder between the polycrystalline CBN compact and the cemented tungsten carbide substrate.
As it was for diamond, the HT/HP sintering process for CBN is effected under conditions in which CBN is the thermodynamically stable phase. It is speculated that under these conditions, intercrystalline bonding between adjacent crystal grains also is effected. The CBN concentration in the compact or in the abrasive table of the supported compact is preferably at least about 50% by volume. Methods for making CBN compacts and supported compacts are more fully described in U.S. Pat. Nos. 2,947,617; 3,136,615; 3,233,988; 3,743,489; 3,745,623; 3,831,428; 3,918,219; 4,188,194; 4,289,503; 4,673,414; 4,797,326; and 4,954,139. Exemplary CBN compacts are disclosed in U.S. Pat. No. 3,767,371 to contain greater than about 70% by volume of CBN and less than about 30% by volume of a binder metal such as cobalt. Such compacts are manufactured commercially by the General Electric Company under the name BZN 6000.RTM..
As is described in U.S. Pat. No. 4,334,928, yet another form of a polycrystalline compact, which form need not necessarily exhibit direct or intercrystalline bonding, involves a polycrystalline mass of diamond or CBN particles having a second phase of a metal or alloy, a ceramic, or a mixture thereof. The second material phase is seen to function as a bonding agent for the abrasive crystal particles. Polycrystalline diamond and polycrystalline CBN compacts containing a second phase of a cemented carbide are exemplary of these "conjoint" or "composite" polycrystalline abrasive compacts. Such compacts may be considered to be "thermally-stable" as compared to metal-containing compacts as having service temperatures above about 700.degree. C. Compacts as those described in U.S. Pat. No. 4,334,928 to comprise 80 to 10% by volume of CBN and 20 to 90% by volume of a nitride binder such as titanium nitride also may be considered exemplary of a thermally-stable material. Such CBN-TiN compacts are manufactured commercially by the General Electric company under the name BZN 8100.RTM..
With respect to supported compacts, it is speculated, as is detailed in U.S. Pat. No. 4,797,326, that the bonding of the support to the polycrystalline abrasive mass involves a physical component in addition to a chemical component which develops at the bondline if the materials forming the respective layers are interactive. The physical component of bonding is seen to develop from the relatively lower CTE of the polycrystalline abrasive layer as compared to the cemented metal support layer. That is, upon the cooling of the supported compact blank from the HT/HP processing conditions to ambient conditions, it has been observed that the support layer retains residual tensile stresses which, in turn, exert a radial compressive loading on the polycrystalline compact supported thereon. This loading maintains the polycrystalline compact in compression which thereby improves fracture toughness, impact, and shear strength properties of the laminate.
In the commercial production of supported compacts it is common for the product or blank which is recovered from the reaction cell of the HT/HP apparatus to be subjected to a variety or finishing operations which include cutting, such as by electrode discharge machining or with lasers, milling, and especially grinding to remove any adherent shield metal from the outer surfaces of the compact. Such finishing operations additionally are employed to machine the compact into a cylindrical shape or the like which meets product specifications as to diamond or CBN abrasive table thickness and/or carbide support thickness. Especially with respect to diamond and CBN supported compacts, a substantially uniform abrasive layer thickness is desirable since the abrasive tables on the blanks are often machined by the user into final products having somewhat elaborate configurations, e.g., sawtoothed wedges, which are tailored to fit particular applications. It will be appreciated, however, that during such finishing operations, the temperature of the blank, which previously had been exposed to a thermal cycle during its HT/HP processing and cooling to room temperature, can be elevated due to the thermal effects of grinding or cutting operations. Moreover, the blank or product finished therefrom may be mounted onto the steel shank of a variety of cutting or drilling tools using braze or solder techniques requiring temperatures of from about 750.degree. to about 800.degree. C. to melt the filler alloy. This again subjects the compacts and supports to thermal gradients and stresses. During each of the thermal cyclings of the supported blank, the carbide support, owing to its relatively higher coefficient of thermal expansion (CTE), will have expanded to a greater extent than the abrasive compact supported thereon. Upon heating and cooling, the stresses generated are relieved principally through the deformation of the abrasive table which may result in its stress cracking and in its delamination from its support.
To improve the bond strength at the interface between, particularly, PCD compacts and their cemented metal carbide supports, proposal have been made to interpose an intermediate layer between the PCD and carbide layers. As is detailed in U.S. Pat. Nos. 4,403,015 and 5,037,704, the intermediate layer is provided to contain less than about 70% by volume of CBN and a balance of a nitride such as TiN, and is sintered directly between the PCD and carbide layers using the traditional HT/HP method. The interposition of a CBN-TiN bonding layer has been observed to prevent the influx or "sweep" of the cobalt binder from the carbide layer to the PCD layer wherein it would have otherwise catalyzed the back-conversion of diamond to graphite hereby weakening the interface between the PCD and carbide layers.
As the supported PCD compacts heretofore known in the an have garnered wide acceptance for use in cutting and dressing tools, drill bits, and the like, it will be appreciated that further improvements in the strength and impact properties of such materials would be well-received by industry. Especially desired would be supported diamond compacts having improved fracture toughness, impact, and shear strengths which would expand the applications for such material by enhancing their machinability, performance, and wear properties. Thus, there has been and heretofore has remained a need for supported PCD compacts having improved physical properties.